1. Field of the Invention
The present invention relates to a lithographic printing plate used in the field of offset printing, and more particularly to a positive- or negative-type original plate for a lithographic printing plate which is used as a so-called computer-to-plate (CTP) plate that is applicable to direct plate making based on digital signals from a computer or the like.
This application is based on Patent Application No. Hei 10-71825 filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
As systems for direct plate making based on digital signals from a computer, there have hitherto been known (1) a system which uses electrophotography, (2) a system which uses Ar laser exposure and post-heating, (3) a system which uses high power laser exposure and post-heating, (4) a system which uses diffusion transfer of a silver salt, (5) a system which ablates a silicone rubber layer by electric discharge or irradiation with a laser beam, (6) a system which forms a mask layer on a conventional lithographic presensitized plate (PS plate) having a light-sensitive image forming layer, forms an image on the mask layer, performs flood exposure, develops, and performs post-treatment to obtain a printing plate, and the like. At present, these have their own merits and demerits, and further investigation is currently under way. Of the above-described systems, particularly the plate material obtained by the technology described in (6) above provides a high resolution image, is adaptable to a large size plate, and provides a printing plate, which is a final product, which is similar to conventional ones, so that printing operators are accustomed thereto through their experience. For this reason or for other reasons, the system has been regarded as a technology replacement system which can be easily accepted by operators at actual operating sites when replacing a conventional plate making system using a film and a PS plate by a computer-to-plate (CTP) plate making system.
As methods for forming a mask image on a conventional PS plate, there have been known i) an inscribing method performed by an ink jet, ii) a thermal transfer method, iii) a method which includes coating a silver salt layer on a PS plate, image-wise exposing the silver salt layer, and developing by a wet method to obtain a mask image, iv) a method which includes ablation of the mask layer provided on a PS plate with a high power laser beam to form an image, and the like.
Of these, the methods i) and ii) have problems that they do not provide high resolution, or the inscribing speed is low, while the method iii) gives high resolution but involves complicated post-treatment or other problems.
The method using ablation with laser beam is a known technology as described in Japanese Patent Unexamined Publications Nos. 50-113307, 50-102401, 50-158405, 53-23705, 53-33702, 58-18290, 60-83893, 61-36750, etc. It has various practical problems. That is, high resolution is difficult to obtain because of non-uniform ablation traces, there is the concern of contamination of appliances due to scattering or evaporation at the time of inscribing, inscribing on a large plate takes a long time because it requires a high energy of usually 800 to 1,200 mJ/cm.sup.2, which makes the inscribing speed very slow, and the like. Also, when images are formed using high energy density light, usually a problem called banding occurs. Banding is a phenomenon which is observed as a band-like pattern, caused by a fluctuation in the output power of a laser device used as an optical source or by a difference in light intensity distribution in an incident plane appearing near the boundary between scanning lines when linear scanning is operated. This phenomenon is observed frequently in the case of directly inscribing on a printing material using a high energy density laser.